3GPP agreed to support ProSe D2D service (Proximity Device-to-Device Service), which includes direct discovery and direct communication. However, at this stage, the study about the access control is still not enough especially the study for the ProSe D2D resource allocation. Take the ProSe D2D service as an example, the ProSe D2D service can be divided into ProSe discovery and ProSe communication, which enable nearby devices to discover and communicate with neighbor devices directly through air links.
The communication service could cover both in-coverage scenario and out-of-coverage scenario, which represents the communication service is achieved in the coverage of long term evolution (LTE) network or out of the coverage of the LTE network. For the convenience of description, we take the ProSe D2D service as an example and use the terminologies ‘direct discovery’ and ‘direct communication’ to represent the ‘in-coverage ProSe discovery’ and ‘in-coverage ProSe communication’ respectively. Next, we will introduce direct discovery and direct communication respectively.
The conceptual graph of discovery resource and discovery signal of direct discovery is shown in FIG. 1. Macro Base Station (MBS) can allocate communication radio resource, e.g. discovery radio resource pool, scheduling assignment message pool, data resource pool etc., in each sub-frame dynamically or semi-persistently. The resource pool is a block of radio resource which is as long as a time period in the time domain and one or more than one frequency subcarriers in the frequency domain. The resource pool may be further divided into multiple resource blocks, which each resource block occupies the same number of time unit and frequency carriers. In each message transmission, UE select one resource block randomly from the resource pool to transmit messages. MBS may provide multiple resource pools for message transmission. There are two resource allocation approaches for direct discovery, Type 1 and Type 2 resource allocation, for MBS to allocate radio resource to User Equipment (UE). In Type 1 resource allocation, MBS allocates the radio resource pool for UEs to transmit signals on a randomly selected resource pool. In Type 2 resource allocation, MBS schedules dedicated radio resource for UEs to transmit signals on the resource pool semi-persistently.
To support direct discovery, transmission and reception of discovery messages should be supported to both radio resource control (RRC) idle mode (RRC_idle) UEs and RRC connected mode (RRC_Connected) UEs. To Type 2 resource allocation, UEs need to stay in the RRC_Connected state to acquire dedicated radio resource from the MBS. Nevertheless, MBS can implement Type 1 resource allocation by broadcasting the configuration of discovery resource pool. Through Type 1 resource allocation, both the RRC_Connected UEs and RRC_idle UEs can realize direct discovery by delivering discovery signals in a contention-based manner.
The conceptual graph of direct communication is also shown in FIG. 1. To direct communication, MBS may allocate communication radio resource semi-persistently. For a UE who wants to deliver communication data, it needs to deliver a ‘scheduling assignment’ (SA) message to indicate the information of the radio resources for interested UEs to receive the following D2D data. Therefore, MBS needs to allocate both the SA radio resource pool and D2D data resource pool to UEs.
There are two resource allocation approaches for direct communication, Mode 1 and Mode 2 resource allocation, In Mode 1 resource allocation, MBS provides dedicated radio resource to UE in the SA pool for UE to deliver SA. UEs need to stay in the RRC_Connected state to acquire and access the dedicated radio resource from MBS. In Mode 2 resource allocation, UE will randomly select the radio resource in the SA pool to transmit its SA and the following data. So, the Mode 2 allocation is a contention-based approach.
The UE may not transmit and receive D2D messages (discovery signals, SA message, or D2D data) simultaneously. The UE plays the role of transmitter UTX when it wants to deliver D2D messages or it plays the role of receiver URX when it wants to receive the D2D messages from neighbor UEs. Furthermore, UE may need to receive all of the receiver (Rx) resource pool and decode every resource block in the Rx resource pool to find out, for example, the D2D messages which it is interested in. UE can switch its role between UTX and URX in every sub-frame or it can also only be UTX or URXpersistently. Also the MBS may assign different radio resource for the transmitter (Tx) pool and the Rx pool to one UE for D2D message transmission and reception respectively.
There are two resource allocation approaches for MBS to allocate SA resource pool and data resource pool. These two approaches are contention-based resource allocation approach and scheduling resource allocation approach which applied to support direct discovery and direct communication. The contention-based resource allocation (Type 1 resource allocation in direct discovery and Mode 2 resource allocation in direct communication) supports both RRC_Connected UEs and RRC_idle UEs to be able to realize in-coverage ProSe D2D service. Connected UE may keep its cellular link with the MBS and it switches to transmit/receive D2D messages in different sub-frames. Idle UE may turn on its RF chain to transmit/receive D2D messages only in some sub-frames which D2D radio resource is provided. contention-based resource allocation approach may have the local congestion phenomenon. To ProSe D2D service, the congestion issue would be more difficult to solve because it becomes a local congestion phenomenon under the coverage of MBS. Such that to solve the possible local congestion issue in a communication network, a communication control mechanism for the resource allocation of communication network services is on demand.